The mollusks
are among the most diverse, and
well-known of the invertebrate groups and include the clams, snails, tusk
shells, chitons, and squids. Nielsen (2001) identifies 5 synapomorphies
that define the phylum: the mantle, the
foot, the radula,
and pectinate gills. The central nervous system is made of a brain
and a pair of longitudinal nerve cords (three in Bivalvia). The brain is
made of three pairs of ganglia that form a ring around the esophagus.

Brusca and
Brusca (2003) suggest that the mollusks arose from within the spiralians before
the advent of segmentation and a sister group relationship with the sipunculans
(they share similar larval characteristics). However, recent
examinations of the Eutrochozoa [e.g. Struck et al. (2007) and Zrzavy et al.
(2009)] suggest that the sipunculids arose from within the annelids; thus any
apparent relationship with the mollusks is superficial.

The relationships between the classes of mollusks are complex and somewhat
contentious. Six different phylogenetic topologies are given by Adamkewicz
et al. (1997). Although molecular studies helped to resolve most of them,
several competing theories of the evolutionary relationships of higher taxa
within the mollusks remain. Sigwart and Sutton (2007) explored two of the
phylogenetic topologies: the Aciculiferan and Testarian models (see Figure
1). The Aculiferan theory (Figure 1-a) suggests that the mollusks are
divided into two unequal monophyletic groups: the Aculifera and the Conchifera.
The Aculifera includes groups that are referred to as the spiny mollusks, that
is, they have a hard skin (cuticula) that has calcareous spicules. The
Aciculifera may have multiple calcareous plates (as in the chitons) but they do
not have large shells that cover the body of the animal. A second
monophyletic clade, the Conchifera, has animals that make shells.

The alternative Testarian theory (Figure 1-b) has the Neomeniomorpha
(Solenogastres) and
Chaetodermomorpha (Caudofoveata) as basal groups with Polyplacophora as a sister
group to all of the shell-bearing taxa. Figure 2, a figure from
Giribet et al. (2006) presents a very different structure of the mollusks.
The interdigitation of conchiferan taxa with aciculiferan taxa and the somewhat
basal position of the scaphopods suggests that the conchiferan structure is
primitive and the loss of shells occurred multiple times (also as suggested by octopuses
and slugs). The Monoplacophora and Polyplacophora, groups with
serially repeated structures in the mollusks, emerge as a monophyletic
group that is sister to one group of the Bivalvia (Pteromorphia). A second
group of Bivalvia (Heteromorphia) emerges as sister to the other
Bivalvia+Polyplacophora-Monoplacophora+Gastropoda. Clearly, the
relationships of the mollusk groups need much more work.

We have chosen to work with a modification of Ruppert et
al. (2004; see Figure 3). Figure 3 most closely approximates the
topology of the Testaria theory (Figure 1-b). The cladogram is modified by
information from Pechenik (2005), Valentine (2004), and Sigwart and
Sutton (2007).

FIGURE 1. This is figure 1 from Sigwart and Sutton (2007). The
main difference between the two views is the monophyly of Neomeniomorpha (Solenogastres),
Chaetodermomorpha (Caudofoveata), and Polyplacophora in the Aculiferan
hypothesis.

FIGURE 2. This is figure 2 from Giribet et al. (2006). Monoplacophora
is nested within the Polyplacophora. Bivalvia emerges as a paraphyletic
group. Scaphopoda, Solenogastres (Neomeniomorpha ), Caudofoveata (Chaetodermomorpha),
and the Cephalopoda make a monophyletic clade.

FIGURE 3. MAJOR
CLADES OF THE MOLLUSCA WITHIN THE PROTOSTOMATA. The cladogram was modified from Ruppert et
al. (2004) and informed by Pechenik (2005), Valentine (2004), Sigwart and
Sutton (2007). Higher taxa within the Mollusca are in bold and within the
shaded box.

The aplacophorans are laterally-compressed
marine worms. Some burrow in the mud or live in interstitial spaces.
Many aplacophorans live among corals and feed on them. They have no shell,
but the mantle, which covers almost all of the body except for a ventral groove,
does have several layers of calcareous bodies. They have a preoral sense
organ and a subterminal ventral mouth. They do not have ctenidia,
but sometimes do have secondary gills. There are no specialized excretory
organs. Some of the aplacophorans do not have a radula. They have
ganglia that are fused with both ventral and
dorsolateral longitudinal nerve cords.

The chitons (Figure 7) are marine animals, which can be
seen on rocks feeding on algae in the intertidal zone. Elongate or oval
and dorsoventrally flattened, chitons are bilaterally symmetrical.
The most obvious distinguishing feature is the dorsal shell of eight overlapping plates embedded in, and
sometimes covered by tissue. They have a large,
muscular, ventral foot and a poorly-differentiated head without eyes or
tentacles. The mantle
cavity is a groove around the foot, with 6-88 pairs of ctenidia, which
together with the overlapping plates, gives it a segmented appearance. The animals feed with a
radula, and the anus
is subterminal. The sexes
are separate and most taxa have larval stages.

The monoplacophorans (Figure 8) are small, deep sea,
snail-like animals. They are almost bilaterally symmetrical with single cap- or
cone-shaped dorsal shell so that they resemble a limpet. The body has a distinct head,
but there are no eyes or sensory
tentacles (except around the mouth). The foot is weakly muscular; anus median, posterior; mantle
cavity large, extending laterally and posteriorly around the foot with 5-6 pairs of
ctenidia; 8 pairs of pedal-retractor muscles; 6 metanephridia; sexes separate;
fertilization external. Only about 20 extant species are known, but many more are
presumed to occur in the deep oceans.

Monoplacophorans are sisters to all other conchiferan
groups (gastropods, cephalopods, scaphopods, and bivalves). Likely, they
resemble the basal organisms that gave rise to all other conchiferan taxa.
Furthermore, they were well represented in the fossil record from the Cambrian
to the Devonian and thought to be extinct. Then, some living animals were
found off the Pacific coast of Costa Rica in 1952 (cited in Ruppert et al.
2004).

The snails are the most speciose group in the Mollusca.
These animals, the snails and slugs are decidedly asymmetrical. The shell
usually is spiraled or coiled and the body of the animal can retract into
it. The body is rotated during development so that the mantle cavity is
anterior (see the explanation under Rhacopodan Clade). There is a single pair of bipectinate ctenidia, but they are
often reduced or lost altogether. The head has eyes and tentacle-like
antennae. The mouth often has jaws as well as a radula. Taxa may be
hermaphroditic or have separate sexes and fertilization may be internal or external.
Also, members of the gastropods may have planktonic larva or direct
development. There are three great groups of gastropods: Prosobranchs,
Opisthobranchs, and Pulmonates.

Prosobranchs: More than half of all snails are prosobranchs whose
defining character is the anterior mantle cavity containing the gill or
ctenidium. Of the more than 20,000 species, most are grazers, but a few
have become carnivores and even parasitic. The prosobranchs are the
sister group to all other gastropods, and they can be identified by the
occurrence of an operculum.

The cone shells (Conus sp., Figure 9) are carnivorous
prosobranchs that can inject powerful toxins into their prey by means of a
specialized hollow radula tooth. Their venoms are under intense study
that may lead to new treatments for pain, depression, and epilepsy.

Opisthobranchs: These are all marine and have a lateral
(or even posterior) mantle cavity. The group includes the sea hares and
sea slugs, groups that have lost their shells. They have limited torsion
during development and a reduction or loss of ctenidia. The nudibranch
sea slugs have no ctenidia and gas exchange occurs across elaborations of the
dorsal surface. At least one species of nudibranch is able to separate
the chloroplasts from the algal cells that it consumes and move them into the
dorsal extensions, thus becoming a photosynthetic animal. Similarly,
some species are able to use nematocysts and move those into the dorsal
extensions as a defensive strategy. Many of the nudibranchs are quite
colorful (e.g. The Spanish Shawl, Figure 10).

Pulmonates: The pulmonate snails are somewhat speciose
(~17,000 species) and most of them are freshwater and terrestrial. They
include the most common snails of streams, ponds, and lakes as well as slugs
and terrestrial snails (e.g. escargot). The most obvious defining
feature is that the mantle cavity serves as a "lung" which can
ventilate both air and water, depending on the species. Some have
evolved gills secondarily as specialized folds of the mantle. The Garden
Snail, Helix (Figure 11), is native to Europe, but it has been
distributed throughout the world where it has become a garden pest in many
places.

Cephalopods (octopods, squids, and nautili) are
bilaterally symmetrical with a linearly-chambered shell that has characteristic
sutures between the
chambers. However, often the shells are reduced or lost. When the external shell
is present, the animal inhabits the last chamber, and a thin filament of living
tissue (the siphuncle) extends through
the older chambers. The head is large and well-defined (Figure 12). It has large, complex eyes and a circle of prehensile tentacles around
mouth, which is equipped with a radula and a beak. The mantle is
muscular and has a large ventral cavity, which contains the gills. The
gill cavity opens to the outside by means of a reduced foot
forming a siphon through which water forced by contraction of mantle, providing jet
propulsion. The sexes are separate, and some tentacles of males are modified for
copulation. Cephalopods are benthic or
pelagic, and entirely marine.

Cephalopods are represented by about 700 species;
however, they were much more dominant in the past. Straight-shelled
nautiloids were some of the most common predators of the Paleozoic. They
were supplanted in the Mesozoic by ammonites (Figure 13), large, generally coiled-shelled,
cephalopods with elaborately developed sutures between the chambers.
Likely, they were the primary food for mosasaurs, and, like the mosasaurs and
non-avian dinosaurs, became victims of the Cretaceous-Tertiary extinction
catastrophe.

Unlike all other mollusks, cephalopods are relatively
intelligent, social animals with elaborate behaviors. Squids and
octopuses can change
color rapidly, which likely is a means of communication, and they perceive
such color changes with nearly vertebrate-like eyes (Figure 14). The intelligence of these animals can
be attested to by anyone who has tried to keep an octopus as a pet.

The Tusk Shells (~500 extant species) are bilaterally symmetrical, with an elongate body in
a tubular one-piece
shell, that is tapered and open at each end (Figure 15).
Often the shell is curved like an elephant-tusk. The mantle
cavity is large and extends along the whole ventral surface. They have
no gills. The head of the Tusk Shell has no eyes, but it has paired clusters of clubbed contractile tentacles
(capitula). The head and cylindrical foot can emerge from the larger end
of the tapered shell. The sexes are separate, and fertilization is
external. All Tusk Shells are benthic and marine.

The Bivalves, also called Pelecypods (~8,000 extant
species), are bilaterally symmetrical benthic freshwater and marine
animals. They have laterally compressed bodies enclosed within two
calcareous, lateral shells, each usually with a beak-like umbo,
hinged dorsally by an elastic ligament and closed by large adductor
muscles. The mantle cavity is large, and the posterior edges of mantle sometimes
fuse to form siphons (incurrent and
excurrent). They have one pair of ctenidia, which are relatively large in most species and used for
filter-feeding. They have almost no head, which has only a mouth with
palps. There are no eyes or radula. The foot is laterally compressed, often greatly
reduced. Some have modified the foot to form a burrowing organ.
Sexes usually are separate and fertilization is external. Larval
stages are aquatic, benthic, sedentary or sessile. The unionid clams with
semi-parasitic larvae called glochidia.

Palaeotaxodonta (Protobranchia): The gills are very small and similar to
those of the gastropods. The primary feeding structures are the palps
that surround the mouth. They probe the mud and capture food particles
with mucus. They also take in sediment and digest food materials of the
organic fraction of the mud. They are among the dominant benthic marine
animals in the deep oceans. The members of the group called the Nut Clams (Nuculids;
ex: Acila, Figure 16) are typical in having a row of short teeth on the
shell.

Cryptodonta: They have elongate shells without hinges
and have gills similar to those of the Palaeotaxodonta. Most are
extinct.

Pteriomorphia: The gills are modified as filters for
food as well as gas exchange organs, the lamellibranch condition. They also are able to secrete a
strong attachment thread, the byssus, that holds the animal to a substrate
surface. Also, most
marine taxa are lamellibranchs.

Oysters (Ostrea and relatives, see Figure 17) include a group of warm water lamellibranchs
that are economically important as food and a source of pearls. They
occur in brackish and freshwater environments and typical lamellibranch in
their form and biology. They are filter-feeders and cement themselves
to any hard surface at hand. Oyster fisheries have suffered in
estuaries like Chesapeake Bay where high levels of sedimentation and
periodic anoxia have caused the local populations to collapse. Food
oysters and pearl oysters are different from each other.

Mussels (Figure 18) also are
economically important as food sources. They occur in colder marine
waters, and attach themselves to substrates by byssal threads. They
can tolerate periodic exposure to air and, therefore, grow in the tidal
zone.

Scallops have shells that have distinctive ridges and
are symmetrical. The animals are benthic but not attached. The
leading edge of the mantle that emerges from the shell has a series of
simple eyes (Figure 19) that allow minimal detection of motion. They can escape
from a potential predator by opening and closing the valves rapidly.

Paleoheterodonta: These are lamellibranch clams that
have teeth in a single row.

Unionid Clams are the most common bivalves in
freshwater. They can be common in certain areas, but they are succumbing
to the requirements of their life cycles. Mature unionids retain the
young in a marsupium until they develop into small larvae that look like
staple-removers called glochidia. The glochidia upon release move
passively in the water until they are pulled into the mouth of a
fish. If it is an appropriate species, the glochidium clamps
onto the gill of the fish and continues to develop. The fish does not
provide nutrients, but brings food particles to the developing larva.
In addition, the fish moves about the stream and allows for the dispersal,
especially upstream, of the clam population. The problem is that fish
which tend to do the most migration (e.g. eels and shad) are impeded by dams
and other human-made obstructions of the stream. Thus, clams without
appropriate host fish, are growing as geriatric populations in many
areas. Some species of unionids evolved marsupia that resemble small
fish. A large fish attacking what it thinks is prey will then get a
mouth full of glochidia.

Heterodonta: These lamellibranch clams have teeth that
are two sizes: short cardinal teeth and long lateral teeth.

These include clams and cockles, many of which are
commercially important. Many can stay under the sand or mud and
reach the water by means of a long siphon. They also include the Giant
Clam (Tridacna, Figure 21) which occur in the tropics of the Indian
and western Pacific oceans and can grow to 200 kilograms.

Zebra Mussels are natives of the Volga Basin in
Russia, but have become invasive exotic animals in North
America. They are lamellibranchs that resemble mussels,
including the obvious byssus and somewhat triangular shape. They
reproduce like most mollusks with a free-swimming veliger larva (Figure 22). The mussels are small, but they do tend to clog water
intakes. They also are quite prolific in that a female can produce up
to one million eggs per year!

Anaomalodesmata: These bivalves have shells that do not
communicate articulate by a hinge. They are similar to lamellibranchs in
their distribution (not in freshwater) and modes of feeding. However,
some have evolved to become carnivorous.